The present invention relates to a superconductive logic device and, more particularly, to a superconductive logic device incorporating Josephson junctions which, for example, can be used for digital computers.
2. Description of the Prior Art
As is well known, a Josephson junction comprises two superconductive layers with a thin insulating layer therebetween, which is so thin that superconductive electrons can pass therethrough. The Josephson junction serves as a switching device, wherein the critical current of the junction is controlled by acting on the junction with a magnetic field.
A first conventional superconductive logic device incorporating Josephson junctions comprises a superconductive ground plane, a superconductive base electrode, a superconductive counter electrode which is opposite to the base electrode, and a superconductive control line. The above-mentioned superconductive elements are electrically insulated from each other. The base and counter electrodes form a Josephson junction in association with a thin insulating layer therebetween. In this case, the control line extends over the junction. The first conventional device is called a magnetic field coupled type gate and is disclosed in J. Matisoo, "The Tunneling Cryotron--A Superconductive Logic Element Based on Electron Tunneling," Proc. IEEE Vol. 55 No. 2 February (1967) pp. 172-180 (in particular, FIG. 3).
The junction has a voltage state and a zero-voltage state. The two states are switched by changing the magnetic field induced by the input signal current flowing through the control line, or by changing the bias current which is supplied from the base electrode and flows through the junction to the counter electrode. Thus, the first conventional superconductive logic device incorporating Josephson junctions can serve as a switching device.
However, in the above-mentioned first conventional device, a large number of layers is required and therefore, a large number of manufacturing steps is required so that the manufacturing cost of the device is high. In addition, since most of the patterns are made by the etching or the lift-off of thin films which are deposited by evaporation or sputtering, a large number of disconnections and short circuits is often created in the stepped portions so that the manufacturing yield of the device is low.
A second conventional superconductive logic device incorporating Josephson junctions comprises a superconductive ground plane, a superconductive base electrode and a superconductive counter electrode, which are also electrically insulated. In this case, the base electrode serves also as a control line. Therefore, the number of layers are reduced and accordingly, the amount of the manufacturing cost is reduced, as compared with the first conventional device. In addition, the number of disconnections and short circuits created in the stepped portions is reduced as compared with the first conventional device. The second conventional device is called a current injection logic type (CIL) and is disclosed in T. R. Gheewala "A 30-ps Josephson Current Injection Logic (CIL)," IEEE J. Solid-State Circ. Vol. SC-14, No. 5, pp. 787-793, October 1979, and in T. A. Fulton, J. H. Magnerlein and L. N. Dunklebeberger, "A Josephson Logic Design Employing Current-Switched Junctions," IEEE Trans. Magn. Vol. MAG-13, No. 1 January (1977) pp. 56-58 (in particular, in FIG. 1).
However, in the second conventional device, since an input signal current is supplied to the base electrode, which also receives a bias current, the input signal current and the bias current are united with each other. Therefore, isolation between input and output components is difficult and accordingly, circuit designs using it are complex.